1. Field of the Invention
The present invention relates in general to a color video printer, and more particularly to optical exposure system and method for a color video printer, in which gradations of colors to be printed by the color video printer can be increased in number so that sensitivity differences among R, G and B colors can be compensated.
2. Description of the Prior Art
In a color video printer, generally, a photosensing process has been employed which records or prints an image by producing an optical image for an electric image signal and exposing a photosensitive medium to a light of the optical image. A cathode ray tube (CRT) has typically been employed in the photosensing process, for production of the optical image for the electric image signal. An optical lens system or an optical fiber has also been utilized in the photosensing process, in order to focus the optical image on the CRT accurately on the photosensitive medium. Noticeably, utilizing the optical lens system or the optical fiber requires a complex system construction with associated equipments, resulting in an increase in manufacturing cost. In this connection, there has been employed a fiber optics liquid crystal display (FOLCD) package which converts the electric image signal into the optical image utilizing a liquid crystal television technique which has recently been developed rapidly, thereby enabling a reduction in system volume and in manufacturing cost.
The FOLCD package is adapted to determine transmitivities of liquid crystal elements by pixels by applying the electric image signal to a liquid crystal display (LCD) panel. In accordance with the determined transmitivities, light from the bottom side of the LCD panel is transmitted to the liquid crystal elements for the conversion of the electric image signal into the optical image to which the photosensitive medium is optically exposed through a screen consisting of the optical fiber.
Referring to FIG. 1, there is shown an example of a conventional optical exposure system for a color video printer in block form. The illustrated system comprises a LCD panel 20 and a LCD controller 10 which inputs R, G and B video signals and outputs gate voltages V.sub.G corresponding to R, G and B colors and video signal voltages V.sub.D corresponding to pixels, to the LCD panel 20. Also, the LCD panel 20 includes R, G and B-line LCDs 21-23 which are in close contact with one another, as shown in FIG. 2.
Referring to FIG. 3, there is shown an equivalent circuit diagram of the LCD panel 20. As shown in this drawing, the LCD panel 20 includes three, R, G and B groups of thin film transistors T.sub.R1 - T.sub.Rh, T.sub.G1 - T.sub.Gh and T.sub.B1 - T.sub.Bh corresponding to the R, G and B-line LCDs 21-23 and to the number of pixels 1-h on one horizontal line. The three groups of thin film transistors T.sub.R1 - T.sub.Rh, T.sub.G1 - T.sub.Gh and T.sub.B1 - T.sub.Bh are connected to three, R, G and B groups of liquid crystal elements L.sub.R1 -L.sub.RH, L.sub.G1 -L.sub.Gh and L.sub.B1 - L.sub.Gh which form corresponding pixels on the R, G and B lines, for driving them. The gate terminals of the thin film transistors T.sub.R1 - T.sub.Rh, T.sub.G1 - T.sub.Gh and T.sub.B1 - T.sub.Bh are commonly connected with one another by lines, in order to receive, by R, G and B lines, the R, G and B gate voltages V.sub.G (V.sub.GR, V.sub.GG and V.sub.GB) from the LCD controller 10. Also, the drain terminals of the thin film transistors R.sub.R1 - T.sub.Rh, T.sub.G1 - T.sub.Gh and T.sub.B1 - T.sub.Bh are commonly connected with one another by pixels, in order to receive, by pixels, the signal voltages V.sub.D (V.sub.D1 - V.sub.Dh) from the LCD controller 10.
The operation of the conventional optical exposure system with the above-mentioned construction will now be described with reference to FIGS. 4A to 4K and FIG. 5.
Referring to FIGS. 4A through 4K, there are shown timing diagrams illustrating the operation of the conventional optical exposure system. First as shown in FIGS. 4A to 4C, the R, G and B gate voltages V.sub.GR, V.sub.GG and V.sub.GB from the LCD controller 10 are in sequence applied to the LCD panel 20, while the signal voltage V.sub.D1 from the LCD controller 10 corresponding to the first pixel on one line is applied to the LCD panel 20 as shown in FIG. 4D. As a result, the thin film transistors R.sub.R1, T.sub.G1 and T.sub.B1 in the LCD panel. 20 are turned on by the R, G and B gate voltages V.sub.GR, V.sub.GG and V.sub.GG from the LCD controller 10 as shown in FIGS. 4A to 4C, so that the signal voltage V.sub.D1 at the drains thereof as shown in FIG. 4D is, by intervals t.sub.R, t.sub.G and t.sub.B, applied to the R, G and B liquid crystal elements L.sub.R1, L.sub.G1 and L.sub.B1 corresponding to the first pixel. Then, the R, G and B liquid crystal elements L.sub.R1, L.sub.G1 and L.sub.B1 maintain their levels respectively during one period T as shown in FIGS. 4E-4G.
As mentioned above, the signal voltages are, by pixels, applied synchronously with the timing of the gate voltages, for driving of the liquid crystal elements by pixels. Finally, when the signal voltage V.sub.Dh from the LCD control let 10 corresponding to the last pixel on one line is applied to the LCD panel 20 as shown in FIG. 4H, the R, G and B liquid crystal elements L.sub.Rh, L.sub.Gh and L.sub.Bh corresponding to the last pixel maintain their levels respectively during one period T as shown in FIGS. 4I-4K, resulting in one line pixel driving completion.
Noticeably, the reason why the signal voltages V.sub.D1 -V.sub.Dh are applied as alternating current (AC) signals having polarities reversed every period is to avoid deteriorations in the liquid crystal elements. Also, the R, G and B liquid crystal elements L.sub.R, L.sub.G and L.sub.B have capacitor effects to determine transmitivities of light from a light source according to the applied signal voltages, for production of the optical image for the electric image signal.
Herein, exposure amount of one pixel of the photosensitive medium to the optical image is determined based on exposure time of the pixel and intensity of the light of the optical image. EQU Exposure amount(Energy)=Exposure time * Light intensity . . . (1)
The exposure time and light intensity are determined respectively based on the timing relation and amplitudes of the signal voltages applied to the LCD panel 20. Namely, the exposure time is determined by a constant period T and the number n of times that the signal voltages are applied to the LCD panel 20. EQU Exposure time=T * n
Also, the light intensity is determined by amount L of back light before transmission and transmitivities determined based on the amplitudes of the signal voltages V.sub.D1 - V.sub.Dh.
Referring to FIG. 5, there is shown voltage-transmitivity characteristics of the liquid crystal elements having eight gradations. From FIG. 5, it is seen that the liquid crystal elements have transmitivities .alpha.1-.alpha.8 of 8 steps according to the applied signal voltages V1-V8. Performing the photosensing for the photosensitive medium by applying the signal voltages V1-V8 to the drains of the corresponding liquid crystal elements, n times, then the R, G and B exposure amounts for respective pixels of the photosensitive medium can be obtained as follows: EQU Exposure amount=T * n * .alpha. * L . . . (2)
where, T: period, n: the number of exposure times, .alpha.: transmitivity and L: amount of back light before transmission to the LCD panel.
It is noted from the above equation (2) that the exposure amount, by R, G and B colors, for respective pixels of the photosensitive medium is of no more than 8 gradations resulting from the transmitivities a of 8 steps (gradations). For this reason, 512 colors can be produced by three, R, G and B colors. EQU 8 * 8 * *=512
It is generally known that, various color expressions more than 64 gradations by R, G and B colors are required to reemerge visually-natural colors. For this reason, the color video printer employing the 8-gradation liquid crystal display could not reemerge visually-natural colors, thereby resulting in degradation in the worth thereof.